My 83SD exhibits symptom of thermostat failure. The bottom hose and radiator were cold while temp gage was just a needle shy of 100 degree C after the car was well driven. I thought beeter to fix it before messing with the boost. Took it out and boiled with a new one (Behr, 80 degree C), they both opened at the same time. I drilled a small hole in the top arraw area of the new thermostat. After installing the new one, the temp gage stayed around 85-87 degree C and does not vary much.

Strange failure mode. The old thermostat is made in France, and it appears to be well made. Probably weak spring to block off the by-pass after years of thermo cycles.

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Shane83SD